1. Field of the Invention
The present invention relates to variable optical attenuators, and particularly to a manual variable optical attenuator having a sealing gasket.
2. Description of Prior Art
Optical attenuators are widely used in optical transmission systems and optical networks. A variable optical attenuator is a passive optical component used to reduce optical power propagating in an optical fiber. The variable optical attenuator can provide a range of attenuations to meet changing demands on the optical network. Variable optical attenuators may be categorized as either electrical or manual. Manual variable optical attenuators have relatively simple structures, and are more widely used in optical transmission systems and optical networks.
Internal optical components of an attenuator must remain clean and dry, to ensure proper operation. Referring to FIGS. 7 and 8, a conventional manual variable optical attenuator comprises a graded transmittance filter, a filter carrier, a screw pole and a housing. The filter is mounted on the filter carrier. An end of the screw pole is rotatably held in a sidewall of the housing, and is exposed to an outside of the housing. A screwdriver is engaged with the end of the screw pole to rotate the screw pole. The screw pole thus drives the filter carrier to shift the filter across a path of light passing through the attenuator. The desired amount of attenuation can thus be obtained. However, a gap exists between the screw pole and the housing at the hole. The gap allows external harmful contamination such as water vapor and dust to penetrate into the attenuator.
An object of the present invention is to provide a manual variable optical attenuator having a gasket for improving sealing.
To achieve the above object, a manual variable optical attenuator in accordance with the present invention comprises a base, a cover, an optical module, a shifting device and an optical fiber retainer. The optical module comprises a mirror, a frame, a dual optical fiber collimator, a stopper, a graded transmittance filter, and a filter carrier. The shifting device comprises a screw pole, a first holder, and a second holder. The screw pole comprises a terminal portion rotatably received in the first holder, a first annular groove accommodating the second holder, a second annular groove receiving a gasket, an adjusting slot, and a screw thread. The gasket protrudes from a circumferential surface of the screw pole and seals a gap between the screw pole and a sidewall of the cover. The gasket thus effectively prevents external harmful contamination such as water vapor and dust from penetrating into the attenuator. Excellent sealing enables the attenuator to operate with minimal insertion loss throughout the lifetime of the attenuator.